I found the pages in al-naHw al-waafiy: p. 43 of volume I. Two and a
half pages. According to him, either kasra or damma is allowed, but a
kasra is more common, except when the second following consontant has a
damma too, as in

أقدم عالمُن ُاخرج لاستقباله = أقدم عالم اخرج لاستقباله

i.e. aqbal @aalimunu _xruj li...etc

He says some claim you can also the tanween altogether if it is followed
by a Harf saakin.